POST request from other domain

Hi,

I am using a custom HTML form to POST some data to the design my night booking system. This all works fine.

When the form is sent we want to then redirect the user back to the squarespace site using:

<script>
  DMN.val('return_url', 'https://www.our-url.co.uk/thank-you');
  DMN.val('return_method', 'post');
</script>

I get the following error:

Uncaught ReferenceError: DMN is not defined
I am told this is because"make sure that your webserver and application can accept POST requests from the designmynight.com domain (including any subdomains)"
How do I go about doing this?

many thanks